Subscriber groups in this case are related, for example, with the feature “Centrex” (Central Office and Exchange). The Centrex feature allows subscriber terminals which are connected to a common telephone exchange, or are each connected to different telephone exchanges, in a telecommunications network via subscriber access lines to be combined to form a Centrex group. The subscribers who belong to the Centrex group may then be provided by the telephone exchange which is responsible for them with virtually the same service features which a private branch exchange (PBX) would offer them if their subscriber terminals were connected to it.
Typical service features which are provided to the Centrex subscribers, and some of which correspond to a private branch exchange are, for example:                access to the telecommunications network by means of an access code,        special charge handling for Centrex-internal connections,        exchange authorizations for incoming and outgoing calls,        call transfer,        call pick up,        service features relating to a call center (attendant features),        secretary and team functions,        various audible signals (“ringing” etc.).        
In order to make it possible to provide the call pick up service feature which has been mentioned, a so-called call pick up group is formed within a Centrex group. A subscriber who belongs to a Centrex group may in this case be associated with a number of different call pick up groups.
The call pick up service feature and further service features which are related to the Centrex groups are described, by way of example, on pages 42 and 43 of a customer brochure “GeoCentrex: The Best Catch for your Network” from Siemens AG, issued in 1995 by Geschäftsgebiet Telekommunikationsnetze, Hofmannstr. 51, D-81359, Munich, Order No. A 308080-X3093-X100-1-7635. This brochure describes three types of call pick up service feature, namely call pick up within a group, directed call pick up and call pick up in conjunction with a night-time service, with the call pick up within a group service feature in practice being that which is most frequently used. According to this, Centrex subscribers are combined to form a Centrex call pick up group. This allows a subscriber who is associated with a call pick up group also to accept an incoming call which is directed to a subscriber terminal associated with that call pick up group at any other subscriber terminal which is associated with that call pick up group rather than on the called subscriber terminal. To do this, the subscriber dials a so-called call pick up access code on the subscriber terminal on which he wishes to take the call. There may also be a number of different call pick up groups within one Centrex group. In a situation where a number of incoming calls occur at the same time within a call pick up group and one subscriber wishes to take a call on another subscriber terminal in the described manner, the call which has already been waiting for the longest time is passed to this subscriber.
Operation of a call pick up service feature is dependent on each subscriber terminal which is associated with the call pick up group, as well as the subscriber, being located within a physical area which allows a call which is directed to a subscriber terminal which is associated with that call pick up group to be perceived audibly or visually by the subscribers. If the subscribers in a call pick up group are not located in such an area, i.e. within audible or visual range, for example because their workstations are accommodated in physically separate offices, then the subscribers cannot audibly or visually perceive an incoming call which is directed to a subscriber terminal which is located in a different office or room, and can thus not pick up the call in the described manner.
This disadvantage is particularly apparent when a call pick up group is set up throughout the network for subscribers whose subscriber terminals are connected to different exchanges. This is because the subscribers in a call pick up group whose subscriber terminals are connected to different exchanges are generally located in a physically separate area. An unfavorable situation such as this also occurs when mobile subscriber terminals in a mobile radio network are associated with such a call pick up group. The mobile terminals are generally not located within audible range of the other call pick up group members. The handling of Centrex service features with regard to subscribers to a mobile radio network and to a landline network has already been proposed in German Patent Application 198 52 774.8.